The browsers will go into
Quirks
mode if a DOCTYPE declaration (DTD) is not included.

The header section.

The header section defined by the HEAD element
(the tag pair <head></head>) contains information about the current document,
such as its title, keywords that may be useful to search engines,
and other data that is not considered document content.

There is a limitation of sub-element types a HEAD element can contain.
You can find which elements in the HTML-reference.

The body section.

The body section contains the document’s actual content. The body section may be
implemented by the BODY element (the tag pair <body></body>)
or the FRAMESET element (the tag pair <frameset></frameset>).

There is a limitation of sub-element types a BODY element can contain.
You can find which elements in the HTML-reference.

There is a limitation of sub-element types a FRAMESET element can contain.
You can find which elements in the HTML-reference.

Example of a simple HTML document with a BODY element:

<!-- We are using HTML 4.01 Strict DTD Version of HTML in this document. --><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN""http://www.w3.org/TR/html4/strict.dtd"><!-- An HTML document is contained by the HTML element. --><HTML><!-- Here comes the header section --><HEAD><!-- the title will appear as part of the title in the browser --><TITLE>This is my first HTML document</TITLE></HEAD><!-- Here comes the body section --><BODY><P>Hello world!</P></BODY></HTML>